Many natural gas and crude oils contain sulfur, as elemental sulfur and as sulfides, polysulfides, mercaptans and other organic and inorganic species. When elemental sulfur deposits as a solid, it can accumulate and result in flow constriction and can reduce the capacity in separation equipment. It can plug instrumentation connections, cause poor process control, and necessitates additional maintenance costs. When elemental sulfur is produced in conjunction with water, the elemental sulfur can be highly corrosive to the carbon steel piping and separation equipment typically used in oil and gas production, transportation and refining operations. The elemental sulfur can also combine with or react with production treating chemicals to form tars and other undesirable solids.
The solubility of elemental sulfur in natural gas is dependent on many factors including the hydrocarbon fluid composition, pressure and temperature of the fluid in the formation, at pressure reduction and cooling systems in the production piping. The solubility of elemental sulfur is strongly dependent on the concentration of other sulfur species such as H2S, and the amount of liquid hydrocarbon associated with gas production. Additionally, solubility may be affected by the volume and salinity of any water produced and the concentration of carbon dioxide in the gas. Solubility of the sulfur may be reduced with reductions in pressure and temperature of the natural gas during movement from the formation into the production, transportation and processing equipment. Air contamination and interaction of sulfide species with oxidized forms of iron may be associated with production of elemental sulfur and thereafter precipitation.
Attempts have been made to use filters to remove the elemental sulfur as if it were a solid particle, but, since it is actually dissolved in the hydrocarbon fluid a filter will not remove the sulfur. However, the pressure drop associated with most solid particulate filters can cause some of the elemental sulfur to deposit on the downstream surface of the filter due to the reduction in solubility caused by the pressure drop of the filter, similar to the pressure drop of a pressure control valve. This does remove a small amount of the elemental sulfur that has dropped out of the solution, but there is still significant soluble and insoluble elemental sulfur remaining in the hydrocarbon fluid.
The prevention of precipitation of elemental sulfur has been the goal of various attempts in the art. These attempts having included actions to prevent oxygen ingress into production and handling operations with the goal of reducing formation of the elemental sulfur from other sulfur species, to designing production equipment with staged pressure drops with the goal of minimizing the potential deposition of elemental sulfur, and to heat the hydrocarbon fluid with the goal of maintaining any elemental sulfur as a dissolved vapor or as liquid elemental sulfur.
Additionally, in order to reduce conduit and equipment plugging in operations, it is typical to provide for the periodic or continuous injection of solvents to remove elemental sulfur deposits or prevent the elemental sulfur from depositing in the system. Solvents used in these operations may be physical solvents (e.g. hydrocarbons or hydrocarbon mixtures, coker gas oil, kerosene/diesel, mineral oil and aromatic solvents such as benzene and toluene) or chemical solvents (e.g. amine based chemicals including aqueous ethylamine and alkyl amines in aromatic solvents, and disulfide based solvents (e.g. dimethyl disulfide)).
The method of application and the amount of solvent are specifically designed or selected for each system. The application of these solvents is not without challenges. In gas production operations the solvents are produced with the gas to the gas plant. For some of the solvents the specific gravity of the solvent loaded with elemental sulfur can be equal to or higher than the specific gravity of the water produced, resulting in separation and handling problems at the gas plant. Some of the solvents can also cause operational problems with the downstream processes. In addition, not adding enough solvent can result in the downstream precipitation of elemental sulfur as the production cools. Each of the solvents has specific handling challenges. The disulfide based solvents have a noxious odor and are very difficult to handle. Coker gas oil has a bad odor and other solvents are linked to environmental, health and/or safety issues. The application of solvents is typically once through. This can result in a large expense associated with sulfur management.
The problem with the deposition of elemental sulfur in the various natural gas and crude oil production facilities and downstream processing equipment and conduits has been observed since at least the 1960's and research has been done to define the levels of elemental sulfur that might be present in hydrocarbon fluids and to help determine where in the system the elemental sulfur might be deposited. As a result, elemental sulfur deposits can become a major problem—especially as coal seam gas and oil shale gas production became a major hydrocarbon resource. Current methods for addressing this problem therefore, appear limited to washing out the elemental sulfur after it has deposited or preventing the elemental sulfur from depositing by tying it up with special solvents.
Other conventional methods do not actually use an adsorbent to remove elemental sulfur, which includes elemental sulfur, polymeric sulfur or zero-valent polymeric sulfur, from hydrocarbon fluids but do propose using an adsorbent to remove non elemental sulfur from hydrocarbon fluids. U.S. Pat. No. 5,686,056, for example, proposes, using a filter media to adsorb and/or break down a hydrogen sulfide—sulfur polymer (H2Sx) to hydrogen sulfide and sulfur, which is collected by the filter media. The hydrogen sulfide-sulfur polymer may be formed during the manufacture of a hydrogen sulfide product from hydrogen and elemental sulfur. As a result, the hydrogen sulfide product stream that was produced (manufactured) is cleaner after the sulfur and H2Sx is removed. The filter media described in the '056 patent, therefore, does not remove sulfur from naturally occurring or processed hydrocarbon fluids but is removing it from a manufactured hydrogen sulfide product stream.